Linux Commands

The Shell - Bash

The shell, or the terminal is a really useful tool. Bash is the standard shell on most Linux distros.

Navigating

pwd - Print working directory

cd - Change directory

cd ~ - Change directory to your home directory

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Looking at files

ls - List files in directory

ls -ltr - Sort list by last modified. -time -reverse

file - Show info about file. What type of file it is. If it is a binary or text file for example.

cat - Output content of file.

less - Output file but just little bit at a time. Use this one. Not more.

Use /searchterm to search. It is the same command as in vim. n to scroll to next search result. Press q to quit.

more - Output file but just little bit at a time. less is better.

A little bit of everything

history - Show commands history

sudo

List what rights the sudo user has.

sudo -l

Sudo config file is usually /etc/sudoers

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Working with files

touch - Create a new file.

cp - Copy

mkdir - Make directory.

# Make entire directory structure
mkdir -p new/thisonetoo/and/this/one

rm - Remove file

# Remove recursively and its content. Very dangerous command!
rm -rf ./directory
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Find

Find is slower than locate but a lot more thorough. You can search for files recursively and with regex and a lot of other features.

# This will send all permissions denied outputs to dev/null.
find / -name file 2>/dev/null

Locate

Locate is really fast because it relies on an internal database. So in order to have it updated you need to run:

sudo updatedb

Then you can easily find stuff like this:

locate filename

Which

Outputs the path of the binary that you are looking for. It searches through the directories that are defined in your $PATH variable.

which bash
# Usually outputs: /bin/bash
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Filters

There are certain programs that are especially useful to use together with pipes. They can also be used as stand-alone programs but you will often see them together with pipes.

sort

sort test.txt

uniq

sort -u test.txt
sort test.txt | uniq
cat filename | sort -u > newFileName

grep

head

tail

tr

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Editing text

sed

Can perform basic editing on streams, that is to say, text.

Remove first line of file/stream

sed "1d"

cut

Cut by column

This is a useful command to cut in text.

Let's say that we have the following text, and we want to cut out the ip-address.

64 bytes from ip: icmp_req=1 ttl=255 time=4.86 ms

cut -d" " -f4

-d stands for delimiter. and -f for field.

tr - Translate

Transform all letter into capital letters

tr "[:lower:]" "[:upper:]" < file1 > file2
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Example Remove character

# Remove characters
cat file.txt | tr -d "."
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# Remove and replace
# Remove all dots and replace them with underscore.
cat file.txt | tr "." "_"

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awk

So awk is an advanced tool for editing text-files. It is its own programming language to it can become quite complex. Awk iterates over the whole file line by line.

This is the basic structure of an awk command

awk '/search_pattern/ { action_to_take_on_matches; another_action; }' file_to_parse

The search pattern takes regex. You can exclude the search portion or the action portion.

This just prints every line of the file.

awk '{print}' filename

Filtering out specific ip-address:

awk '/172.16.40.10.81/' error.log

Now we want to print out the fourth column of that file, we can just pipe this to cut, but we can also use awk for it, like this:

awk '/172.16.40.10.81/ {print $4}' error.log
# Another example
awk '{print $2,$5;}' error.txt
This prints columns 2 and 5.

We can use the -F flag to add a custom delimiter.

awk -F ':' '{print $1}' test.txt

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3. User management

To add a user we do:

adduser NameOfUser
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# On some machines it is
useradd nameOfUser

To add user to sudo-group:

adduser NameOfUser sudo

On some machines we might not be able to edit the sudoers file because we don't have an interactive shell, in this case can you can just redirect the text into the file, like this:

echo "username ALL=(ALL) ALL" >> /etc/sudoers

Check which users are in the sudo group:

cat /etc/group | grep sudo

Switch user in terminal:

su NameOfUser

Remove/delete user:

sudo userdel NameOfUser

4. Permissions

ls -la

Shows all the files and directories and their permission settings.

drwxrwxrwt 2 root root 4,0K ago  3 17:33 myfile

Here we have 10 letters in the beginning. The first one d shows that it is a directory. The next three letters are for read, w for write and x for execute. The first three belong to the owner, the second three to the group, and the last three to all users.

​https://linuxjourney.com/lesson/file-permissions​

5. Processes

To display information regarding the systems processes you can use the ps command.

ps -aux

-a stands for all -u stands for all processes by all users -x stands for all processes that don't run a tty+

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Install package

Example of how to install something with apt:

sudo apt-get install nmap
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Remove packages

This can be tricky. First find the package

dpkg --list

Then you find it in your list.

sudo apt-get --purge remove nameOfProgram

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Organizing your $path variable

I am talking about debian/ubuntu here. On other systems I don't know.

You can define your path in /etc/environment. If you don't have it you can create it and add the path like this:

source /etc/environment && export PATH

If you are using zsh (which you should) you have to add it here

sudo vim /etc/zsh/zshenv

And add this line somewhere:

source /etc/environment

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Adding a path

This is a non-persistent way to add binaries to your path. Might be useful if you have entered a system that has limited binaries in the path.

export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

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7. Cronjobs

There are two ways to configure cronjobs. The first one is by putting scripts in the following folders.

/etc/cron.daily
/etc/cron.hourly
/etc/cron.weekly
/etc/cron.monthly

The second way is to write the command in the crontab

# list cronjobs
crontab -l
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# Edit or create new cronjobs
crontab -e

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8. Devices

List all devices

fdisk -l

9. The Filesystem

Difference between sbin and bin

sbin is system binaries. A normal user do not have access to these binaries. It is only root and users with sudo privileges that do.

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Mount

So everything on the linux-filesystem belongs to some part of the filesystem-tree. So if we plug in some device we need to mount it to the filesystem. That pretty much means that we need to connect it to the filesystem. Mount is like another word for connect.

So if you want to connect a CD-rom or USB to your machine. You need to mount it to a specific path on the filesystem.

So if you plug in the usb it might be accessible at /dev/usb. But that it not enough for you to be able to browse the usb content. You need to mount it. You do this by writing

mount /dev/usb /media/usb

Or whereever you want to mount it.

So when you click on Eject or Safetly remove you are just unmounting.

umount /media/usb

Knowing how to mount and unmount might be useful if you want to get access to a remote NFS-directory. You will need to mount it to your filesystem to be able to browse it.

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10. Controlling services

Systemctl

Systemctl can be used to enable and disable various services on your linux machine. Start ssh

systemctl start ssh
systemctl status ssh
systemctl stop ssh

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You can verify that the service is listening for connection by running network status.

netstat -apnt

Make ssh start upon boot

systemctl enable ssh
systemctl enable apache2

Init.d

Init.d is just a wrapper around Systemctl. I prefer it.

/etc/init.d/cron status
/etc/init.d/cron start
/etc/init.d/cron stop

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16. Network basics

Netstat - Find outgoing and incoming connections

Netstat is a multiplatform tool. So it works on both mac, windows and linux.

$ netstat -antlp

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Find out what services are listening for connection on your machine Flags

-a # All
-n # show numeric addresses
-p # show port
-t # tcp

netstat -anpt

To easily check out what process is using lots of bandwidth you can use nethogs.

sudo apt-get install nethogs
nethogs

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Firewall - Iptables

Iptables is a firewall tool in linux. A firewall is basically a tool that scans incoming and/or outgoing traffic. You can add rules to the iptables to filter for certain traffic.

Types of chains

So you can filter traffic in three different ways input, forward, and output. These are called three different chains.

INPUT This is for incoming connections. If someone wants to ssh into your machine. Or a web-server responds to your request.

FORWARD This chain is used for traffic that is not aimed at your machine. A router for example usually just passes information on. Most connections are just passing through. As you can see this will probably not be used so much on your machine, as a normal desktop or a server doesn't router that much traffic.

OUTPUT

This chain is used for outgoing traffic.

Active rules

To view your active rules you do

iptables -L
# It will output something like this
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Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
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Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
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Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

So as we can see the current policy is to accept all traffic in all directions.

If you for some reason has been tampering with the iptables and maybe fucked up. This is how you return it to the default setting, accepting all connections

iptables --policy INPUT ACCEPT
iptables --policy OUTPUT ACCEPT
iptables --policy FORWARD ACCEPT

If you instead want to forbid all traffic you do

iptables --policy INPUT DROP
iptables --policy OUTPUT DROP
iptables --policy FORWARD DROP

Okay, so let's block out some connections. To do that we want to add/append a new rule. We want to block all connections from our enemy 192.168.1.30.

# A for append, and S for source. 
iptables -A INPUT -s 192.168.1.30 -j DROP
# Block an entire range
iptables -A INPUT -s 192.168.1.0/24 -j DROP

Now if we want to see our current rules we just do

iptables -L

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Measuring bandwidth usage

There are a few different tools in hour arsenal that we can use to measure bandwidth usage. We will start with iptables.

To view the input and output traffic we just list the rules with some verbosity.

iptables -L -v
# Stdout
Chain INPUT (policy ACCEPT 6382 packets, 1900K bytes)
 pkts bytes target     prot opt in     out     source               destination         
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Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
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Chain OUTPUT (policy ACCEPT 4266 packets, 578K bytes)
 pkts bytes target     prot opt in     out     source               destination

So clean this up and reset the count we can do the following

# Restar the count
iptables -Z
# Remove all the rules, FLUSH them
iptables -F

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Examples

Block outgoing connections to a specific ip

iptables -A OUTPUT -d ip -j DROP